Answer: because of lesser density than water.
Explanation:
The substances which have lesser density than water will float in water and the substances which have greater density than water will sink in water.
The density of water is liquid state is more than the density of water in solid state. Water has a property to expand when it freezes and it forms cage like structure in the solid state. This results in the increase of volume and a corresponding decrease in density.
Thus ice floats in water.